The value of solar batteries as clean energy sources has been acknowledged, and the demands therefor are growing. Solar batteries can be used in a wide variety of fields, from power energy sources for large-scale devices to small-sized power sources for precision electronic devices. In order for solar batteries to be widely used in various fields, the properties of the batteries, especially the output characteristics thereof, need to be accurately measured. Otherwise, it is assumed that various problems may occur in the devices using the solar batteries. Therefore, a technology that can radiate high-precision artificial sunlight, which can be used for inspecting, measuring, and testing a solar battery, over a wide area is particularly in demand.
The main element required in artificial sunlight is to make the emission spectrum of the artificial sunlight analogous to that of reference sunlight (established by Japanese Industrial Standard) and also to make the illuminance of the artificial sunlight about the same level as that of reference sunlight. Artificial sunlight radiation apparatuses are being developed as apparatuses that can radiate such artificial sunlight. Normally, such an artificial sunlight radiation apparatus radiates artificial light (artificial sunlight) with uniform illuminance onto a light-receiving surface of a solar battery so as to be used for measuring, for example, the amount of electricity generated in the solar battery.
An artificial sunlight radiation apparatus (solar simulator) discussed in Patent Literature 1 has a structure that returns light output from a lamp to a radiation surface by using a reflective plate and has an optical filter, which is for forming an artificial sunlight spectrum, disposed between a surface where a measured object is set and the reflective plate. In the structure according to this related art, the reflection angle of the reflective plate is set such that the light output toward the reflective plate from the lamp changes direction and travels toward the radiation surface, whereby uniform illuminance is maintained at the radiation surface.